310 \noindent

   310 \noindent

   311 but often just write {\it hello}.

   311 but often just write {\it hello}.

   312 

   312 

   313 If you prefer to think in terms of the implementation

   313 If you prefer to think in terms of the implementation

   314 of regular expressions in Scala, the constructors and

   314 of regular expressions in Scala, the constructors and

   316 

   317 

   317 \begin{center}

   318 \begin{center}

   318 \begin{tabular}{rcl}

   319 \begin{tabular}{rcl}

   319 $\varnothing$ & $\mapsto$ & \texttt{NULL}\\

   320 $\varnothing$ & $\mapsto$ & \texttt{NULL}\\

   320 $\epsilon$    & $\mapsto$ & \texttt{EMPTY}\\

   321 $\epsilon$    & $\mapsto$ & \texttt{EMPTY}\\